The present invention relates to a clock generator and a mobile communications device using the same, and more particularly, to a single-crystal clock generator capable of operating in different power modes to generate different frequency clock signals, and a mobile communications device using the same.
With the development of electronic technologies, a mobile communications device may be equipped with more than one wireless communications service, such as GSM/GPRS/EGPRS (GGE), Bluetooth, Wireless Fidelity (Wi-Fi), Worldwide Interoperability for Microwave Access (WiMAX) wireless communications service, Long Term Evolution (LTE) wireless communications service, and so on. A modern mobile electronic device may contain many wireless communications blocks to provide communications services corresponding to different wireless communications standards, respectively. In addition, for reducing the production cost, all the wireless communications blocks of the mobile electronic device may share a single reference oscillator, since the cost of a precise oscillator (e.g. a crystal oscillator) is very high.
Generally, a mobile communications device requires multiple external crystal oscillators to provide reference clocks for different functional blocks of the device. For example, a transceiver chip of the communications device may require an external high-frequency (e.g. 26 MHz) crystal with high performance specifications to provide a reference clock for radio frequency (RF) signal generation or modulation/demodulation operations for various communications services such as GSM/GPRS/EGPRS (GGE), Bluetooth, Wireless Fidelity (Wi-Fi), Worldwide Interoperability for Microwave Access (WiMAX), Long Term Evolution (LTE), and so on. Additionally, the mobile device may require another low-frequency (e.g. 32 KHz) crystal for providing a reference clock during mobile communications standby (e.g. Real-time clock, RTC) or during battery-off (when the battery is removed from the device).
However, the cost of crystal oscillators is prohibitively high. For example, for a mobile communications chip with a production cost of around 1-2 USD, the high-frequency crystal (e.g. costs around 0.1 USD) and the low-frequency crystal (e.g. costs around 0.15 USD) together may account for up to 20% of the production cost of the whole chip. Also, the area of printed circuit board (PCB) required by the device would also be increased due to the use of multiple oscillators.
Therefore, it has become a common goal of the industry to provide a way of clock generation for mobile communications devices while reducing chip cost and circuit area.